Effects of low cycle impact fatigue on the residual mode II fracture energy of adhesively bonded joints

Abstract

Cyclic loading exerts a profound influence on the durability of adhesively bonded joints, particularly under cyclic impact loads. Under normal cyclic loading, adhesive joints typically reach their fatigue limit when subjected to approximately 50% of their maximum static strength. However, the endurance limit is considerably lower for cyclic impact loading, sometimes even falling below 5% of the impact strength. This paper investigates the effects of low-energy cyclic impacts on the fracture energy of an end-notched flexure (ENF) joint bonded with an epoxy-based adhesive. The study includes an analysis of the static fracture energy of the ENF joints prior to subjecting them to repeated impact cycles. Additionally, certain joints were subjected to repeated impact cycles until complete crack propagation occurred. To evaluate the residual critical fracture energy of these joints, certain cyclic impact tests were interrupted, and the joints were subsequently subjected to quasi-static loading conditions. The results reveal the remarkable sensitivity of the fracture energy of adhesive joints to the impact energy level, challenging the assumption of infinite life under cyclic impacts often made in conventional fatigue tests. Stress concentration caused by cyclic impact stress waves leads to a higher density of cracks at the specimen edges. The findings are supported by numerical analysis conducted in this study. These results underscore the importance of comprehensive inspections for bonded structures exposed to low-energy cyclic impacts, as impact fatigue can significantly diminish joint strength. The fracture energy decreases as the number of impact cycles increases, indicating a cumulative effect. The findings contribute to a better understanding of the mode II fracture behavior of adhesively bonded joints under repeated impact loads and provide insights for safe design and inspection practices.